1. Field of the Invention
This invention relates to a thermoplastic elastomer composition for powder slush molding and a process for the preparation of that composition. More specifically, the invention relates to a thermoplastic elastomer composition having a high degree of fluidity in a powdery state and a low melt viscosity, and which may enable the easy release of a molded skin from a mold.
2. Background Art
Powder slush molding processes are widely employed in the automotive industry to form the skins of instrument panels, console boxes, door trims, and other interior trims in automobiles. The skins are soft to the touch, allow for leather graining and stitching, and permit great design freedom. Unlike injection or compression molding, no pressure is applied for shaping in powder molding processes. Therefore, the powdery material for powder slush molding ideally should not only have a high degree of fluidity in the powdery state so as to attain the uniform adhesion of the material to a mold having a complicated shape, but should also exhibit a low melt viscosity so that it can be molten and fluidized without pressing to form a film. Furthermore, it is desirable that the skin layer formed on a mold release easily from the mold after cooling.
A powder material composition designed to meet these goals has been proposed in Japanese Pat. Appln. Laid-Open Gazette No. Hei 7-82433. Disclosed therein is a powder slush molding material prepared by pulverizing a mixture made up of a polypropylene resin and a specified styrenic thermoplastic elastomer at a weight ratio between 70:30 and 30:70. The styrenic thermoplastic elastomer is chosen from among styrene/ethylene/butylene/styrene block copolymers, styrene/ethylene/propylene/styrene block copolymers, and hydrogenated styrene/butadiene rubbers, with all having a styrene content of no more than 20% by weight. Compositions with each of these elastomers are suitable for powder molding.
However, the composition made up of the polypropylene resin and a hydrogenated styrene/butadiene rubber is particularly problematic in that the oligomeric component contained in the composition bleeds to the surface because of the poor oil absorbing power of the hydrogenated styrene/butadiene rubber. This makes the composition sticky, causing aggregation and poor fluidity in the powdery state. This also makes it difficult to release skins from molds.